Copy model vertex buffer position data to array

1

Do I need to use a struct with the exact size of the vertex buffer’s vertex stride when copying the vertex buffer data to an array to access the posittions?
Is there some way I can extract only the positions? This would be simpler because I would not need to know the vertex declaration beforehand, define a matching vertex format and use this as the array’s data type…

2

No, but it’s faster if the two match.
(Unless it runs in 32bit mode on an AMD Phenom. Then it’s painfully slow)

1 Like
3

Is that for a tool or a game?
ES GL (Android) doesn’t support getting data from buffers.

Also this is generally a slow operation. if you plan to update a buffer on every frame you can create a shadow copy initially to speed thing up.

Also also, if this is about manipulating Models look at my RawModelProcessor and DynamicModelProcessor at Aether.Extras

4

I just wanted to visualize the teapot thread normals and I wanted to compare the normals from the VertexBuffer against normals calculated from the positions.

5

Ok, then you can get the normals in a Vector3[] as well.
A good example is the GetTextureCoordinate test. You have to pass an offsetInBytes that points at the Normal element.

1 Like
6

It worked, I adjusted it and could read the positions to a Vector3[] :slight_smile:

7

You might find this helpful as well. I use it to read indices into pairs of three (one triange).

        //convert indexed, to non indexed
        Vector3[] tmpVertices = new Vector3[vnum];
        int sizeOfEachVertexInBytes = vertexBuffer.VertexDeclaration.VertexStride;
        vertexBuffer.GetData(baseVertex * sizeOfEachVertexInBytes, tmpVertices, 0, vnum, sizeOfEachVertexInBytes);

        TriangleIndices16[] tmpTriangles = new TriangleIndices16[primitiveCount];
        indexBuffer.GetData(startIndex * 2, tmpTriangles, 0, primitiveCount);

        for (int i = 0; i < primitiveCount; i++)
        {
            Vertices[i * 3 + 0 + vbase] = tmpVertices[tmpTriangles[i].A];
            Vertices[i * 3 + 1 + vbase] = tmpVertices[tmpTriangles[i].B];
            Vertices[i * 3 + 2 + vbase] = tmpVertices[tmpTriangles[i].C];
        }

struct TriangleIndices16
{
    public Int16 A, B, C;
    public int VertexStride { get { return (sizeof(Int16)) * 3; } }

    public static TriangleIndices32 operator +(TriangleIndices16 left, int value)
    {
        TriangleIndices32 result;
        result.A = (Int32)(left.A + value);
        result.B = (Int32)(left.B + value);
        result.C = (Int32)(left.C + value);
        return result;
    }

    public override string ToString()
    {
        return String.Format("A:{0,4} B:{1,4} C:{2,4}", A, B, C);
    }
}

internal struct Triangle : IEquatable<Triangle>
{
    public Vector3 A;
    public Vector3 B;
    public Vector3 C;

    public Triangle(Vector3 A, Vector3 B, Vector3 C)
    {
        this.A = A;
        this.B = B;
        this.C = C;
    }

    public Triangle(ref Vector3 A, ref Vector3 B, ref Vector3 C)
    {
        this.A = A;
        this.B = B;
        this.C = C;
    }

    public Vector3[] GetCorners()
    {
        return new Vector3[] {A,B,C};
    }

    public BoundingBox GetBoundingBox()
    {
        return BoundingBox.CreateFromPoints(GetCorners());
    }

    public Plane GetPlane()
    {            
        return new Plane(A,B,C);
    }

    public Vector3 GetNormal()
    {
        return GetPlane().Normal;
    }
    
    public bool Equals(Triangle other)
    {
        return (A==other.A && B==other.B && C==other.C);
    }

    public static bool operator !=(Triangle a, Triangle b) { return !a.Equals(b); }
    public static bool operator ==(Triangle a, Triangle b) { return a.Equals(b); }

    public static Triangle Zero { get { return new Triangle(Vector3.Zero, Vector3.Zero, Vector3.Zero); } }
}
2 Likes